In the year 140 AD, twenty years after the Ninth Legion disappeared in the north of Britain, Marcus Flavius Aquila, a young Roman centurion, arrives in Britain to serve at his first post as a garrison commander. Aquila's father was the last to hold the eagle standard
of the ill-fated legion, and Aquila hopes to redeem his family's honor
by bravely serving in Britain. Shortly afterwards, only Aquila's bravery
saves the garrison from being overrun by Celtic tribesmen. He is
decorated for his bravery but honorably discharged due to a severe leg
injury.

Living at his uncle's estate near Calleva
in southern Britain, Aquila has to cope with his military career having
been cut short and his father's name still being held in disrepute.
Heeding rumors that the eagle standard has been seen in the north of
Britain, Aquila decides to recover it. Despite the warnings of his uncle
and his fellow Romans, who believe that no Roman can survive north of Hadrian's Wall, Aquila travels north into the territory of the Picts, accompanied only by his British slave Esca. Esca, the son of a Brigantes chieftain, detests Rome and what it stands for, but also considers himself bound to his master, who saved his life during an amphitheater show.

After several weeks of traveling through the northern wilderness,
Esca and Aquila encounter Guern, one of the survivors of the Ninth
Legion, who attributes his survival on the hospitality of the Selgovae
tribe. Guern recalls that all but a small number of deserters were
killed in an ambush by the northern tribes — including Esca's Brigantes —
and that the eagle standard was taken away by the Seal People, the most
vicious of the tribes. The two travel further north until they are
found by the Seal People. Identifying as a chieftain's son fleeing Roman
rule and claiming Aquila as his slave, Esca is welcomed by the tribe.
After allowing the Seal People to mistreat Aquila, Esca eventually
reveals that his actions were a ploy and helps his master to find the
eagle. As they retrieve it, they are ambushed by several warriors,
including the Seal Prince's father, but Marcus and Esca manage to kill
them and escape with the eagle standard. With the aid of the Seal
Prince's young son, Esca and Marcus manage to escape the Seal People's
village.